The present invention relates to tunable filters for electronic communication devices and apparatus and more particularly to high quality factor dielectric core tunable filters for microwave frequencies of from 0.5 to 30 GHz.
Tunable microwave filters have traditionally served a variety of niche applications in military and civilian systems, but recent advances in software defined radio have opened up significantly larger markets for this technology. Signals intelligence, or SIGINT, is the gathering of transmitted radio signals. It is considered essential to the conduct of all large scale military operations, and is widely used in identifying and locating targets. Traditional military applications include wide band receivers for SIGINT systems and electronic warfare. Commercially, these filters have primarily been used in the test and measurement equipment.
Electronic warfare is a fairly broad category that covers everything from jamming of communication and radar signals to radar spoofing and electronic attack. In general, electronic warfare systems require very rapid tuning (typically under one microsecond). In addition, many of these filters are used on transmit and therefore require significantly higher power handling. On transmit, insertion loss is a critical parameter, but isolation can often be relaxed so that lower quality factor technologies can be used. Currently this market is dominated by varactor tuned filters. In the late 1990′s the concept of software radio began to move from a research concept to a widely deployed technology. Software radio offered the promise of multi-band, multi-function, and multi-mode radio systems. To date, software radio has delivered reasonably well on both the multi-function and multi-mode aspects, but has not truly delivered on the multi-band promise. Tunable filters are one of the technologies that have limited the success of software radios ability to provide multi-band solutions.
Current filter technologies are directed to Yttrium Iron Garnet (YIG), varactor, and ferroelectric types of filters. YIG filters are recognized as tunable filters that provide multi-octave tuning, low loss, and high selectivity. However, YIG filters are also recognized to have limited power handling, relatively poor linearity, slow tuning speeds, high drive power requirements, and poor thermal stability. Based on the advantages of YIG filters, it should not be surprising that they are widely used in Signals Intelligence (SIGINT), but are not suitable for many Electronics Warfare (EW) applications. Varactor tuned filters are recognized for their high speed tuning, but they have low quality factors and therefore can offer low loss or high selectivity, but not both. These filters are commonly found in applications such as EW where high speed tuning is critical. Recently, tunable filters fabricated using ferroelectric thin films (primarily barium strontium titanate) have been developed. These filters have shown higher quality factors compared to varactor filters and higher speed than YIG filters.
Other filters include the combline filter which is a widely used filter implementation. Typical combline filters are formed from machined aluminum parts. Resonators are formed from square or circular cavities with posts protruding up from the base. Typically the posts are circular and are enclosed in square cavities. The filters are commonly plated with silver to reduce the losses. The vast majority of these filters are not tunable.
Ceramic filters and resonators are also known and widely used in industry. These filters are typically made from multiple (2-8) individual resonators that are coupled together via an external circuit.
Barium strontium titanate (BST) filters use a combline structure that is tuned and loaded by a capacitor fabricated with barium strontium titanate. One example can be found in U.S. Pat. No. 6,801,104, entitled “Electronically Tunable Combline Filters Tuned by a Tunable Dielectric Capacitor”.